OrtliebSeatpost Bag 4L

Clamping easily to your seatpost with a simple strap and clip (no tools required!), the Seatpost Bag gives you 4 litres of carrying capacity for tubes, tools and other bits for a day out, plus an external bungee drawcord for attaching bulkier items. Inside, a couple of mesh pockets help keep smaller essentials from getting buried.

The fabric, construction and closure are all waterproof, so you needn't worry about whatever you're carrying, and because it sits between you and the rear wheel, it offers a bit of protection from mud and spray too.